In case you have completely run out of 3.5", and 5.25" drive bays to cram in the last of your hard drives, and have a few slots next to your RAID card left, you would probably like this enclosure. Novac designed what it calls "PCI mounting kit", that uses one expansion card slot to hold a 2.5" HDD. What's more, it even provides a door that allows users to add or remove the drive from behind the case, without having to remove it. This perhaps is also its biggest USP, as 2.5" drives, especially SSDs, are very pocketable. Novac plans to sell this for about $36 a piece.

And it'd also depend on how worried you are about the temps of SSDs. They're currently in the 40º-45º range; I don't know what their thermal limit is but I'd be confident they could handle another 10º.